Question: Which of these numbers is prime? ${29,\ 55,\ 57,\ 64,\ 85}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 29 are 1 and 29. The factors of 55 are 1, 5, 11, and 55. The factors of 57 are 1, 3, 19, and 57. The factors of 64 are 1, 2, 4, 8, 16, 32, and 64. The factors of 85 are 1, 5, 17, and 85. Thus, 29 is a prime number.